1. Technical Field
The invention relates to a strain sensor for optical fibers, preferably in relation to tunable optical wavelength filtering in fiber optic based systems.
2. Background Art
Filters based on fiber Bragg gratings are finding increasing use for wavelength selection and de-selection in fiber optic based systems. The theory and construction of these filters are well known, see for example Andreas Othonos, Kyriacos Kalli xe2x80x9cFiber Bragg Gratings. Fundamentals and Applications in Telecommunications and Sensing.xe2x80x9d Published by Artec House Inc. 1999, ISBN 0-89006-344-3. In such filters the refractive index of a section of optical fiber varies periodically along its length, giving regions of alternate high and low index. Light whose wavelength is twice the optical spacing between successive regions of high index will be reflected back along the fiber, whereas all other wavelengths will be transmitted. The bandwidth of the reflected light will depend on the length of modified fiber (the length of the fiber Bragg grating); typically the bandwidth is given by the wavelength divided by the number of consecutive high/low index regions. In practice, fiber Bragg grating lengths can be a millimeter or two up to several tens of millimeters depending on the application.
A filter so described will reflect at a fixed wavelength, but it is often convenient to be able to vary the reflected wavelength. This can be achieved by varying the optical spacing of the grating, either by varying the refractive index of the fiber material or by straining the fiber to alter its physical length. Many such devices are known. For example Limberger H. G., Iocco A., Salathxc3xa9 R. P., Everall L. A., Chisholm K. E., Bennion I. xe2x80x9cWideband Tuneable Fibre Bragg Grating Filters.xe2x80x9d ECOC ""99 September 1999, Nice, France; and Alessandro Iocco, Hans Georg Limberger, Renxc3xa9 Paul Salathxc3xa9, Lorna A. Everall, Karen E. Chisholm, John A. R. Williams, Ian Bennion. xe2x80x9cBragg Grating Fast Tunable Filter for Wavelength Division Multiplexing.xe2x80x9d Journal of Lightwave Technology, Vol. 17, No. 7, July 1999, describe systems using piezoelectric actuators to stretch or compress a fiber Bragg grating and thus alter its reflected wavelength. In an alternative known arrangement described in Ball G. A., Morey W. W. xe2x80x9cCompression-tuned single-frequency Bragg grating fiber laser.xe2x80x9d, Optics Letters, Vol. 19, No. 23, Dec. 1, 1994, stepper motors and lead-screws are used to achieve strain variation. U.S. Pat. No. 5,007,705 issued Apr. 16, 1991 and assigned to United Technologies Corporation achieves refractive index variation by various means including temperature, or strain variation. The contents of these references are incorporated herein by reference to the extent that they teach the background art of tunable fiber Bragg gratings.
In Ball et. al. referenced above, the arrangement for stretching or compressing a fiber described is shown schematically in FIG. 1a. An optical fiber 10 has a Bragg grating formed in it in the region 15. Ferrules 11 and 14 are bonded to the fiber by adhesive 16 and 17 and either or both of them may be moved parallel to the axis of the fiber by a stepper motor (not shown). In alternative known arrangements this can bea piezoelectric device. The motion can be arranged to compress or to stretch the fiber. It may be noted that compression is desirable as the fiber is stronger in this mode than extension and greater strain and thus tuning range can be induced. To limit buckling or distortion during compression additional ferrules 12 and 13 are provided which are not bonded to the fiber such that the fiber can move freely in them in a direction axial to the fiber. Additionally ferrules 12 and 13 can slide in a xe2x80x9cVxe2x80x9d groove or other guiding mechanism such as a parallel spring strip mechanism (not shown). Two ferrules are shown though three or more can of course be used dependent on the length of Bragg grating required. Each one is typically 9 mm long, this length of ferrule being available commercially, but obviously other lengths and diameters could be used.
FIG. 1b shows schematically an arrangement described in U.S. Pat. No. 5,007,705 for straining a fiber whereby the fiber 10 containing the Bragg grating section is wrapped around a piezoelectric cylinder 18. The cylinder expands or contracts radially on application of a suitable voltage V between the inner and outer surfaces 18 of the cylinder, which are metallised, thus straining the fiber 10.
In all tunable systems so described it is desirable that the reflected wavelength be a unique and linear function of some control command parameter, generally a voltage. However this is not achievable in systems using mechanical or piezoelectric means to affect a change in strain. Piezoelectric actuators suffer from non-linearity and hysteresis which means that if a given voltage V is approached from a previous value less than V the resultant reflected wavelength will be different from the reflected wavelength obtained if V were approached from a previous value above V. Also in lead-screw or other mechanical systems hysteresis in the form of backlash gives the same undesirable effect. Furthermore, in the known mechanical and piezoelectric systems described above, force is exerted on the fiber via a ferrule or other coupling bonded to the fiber using an adhesive. This adhesive layer introduces further hysteresis.
One known approach to compensating for piezoelectric actuator hysteresis and non-linearity is by using a strain gauge on the actuator to measure the actuator extension and provide feedback to a control system that maintains the required extension, as described in Limberger et al. and locco et al referenced above. This reduces the problem of piezoelectric actuator hysteresis but strain gauges themselves are not perfectly hysteresis free and problems still persist due to hysteresis in the adhesive bond and any other mechanical linkage between the actuator and the fiber.
It is an object of the invention to avoid or mitigate problems in known systems. It is a further object to provide a system for tuning a fiber Bragg grating in which the reflected wavelength is essentially a unique and linear function of the control command parameter.
According to the invention there is provided an optical fiber strain measurement apparatus comprising an optical fiber, at least one actuator for exerting a force to strain the fiber by compression or extension, and a sensor arranged to sense an absolute value of the strain of the fiber. Because an absolute value is sensed rather than merely monitoring the movement of the actuator itself, as in prior art arrangements, enhanced accuracy is achieved. The actuator and sensor form a closed loop whereby a signal representative of the absolute value controls the actuator to achieve a desired strain. As a result the value of strain desired can be compared with that obtained to control the force exerted by the sensor to obtain the exact desired absolute value.
In a preferred system the measurement apparatus is used with a Bragg fiber grating such that the grating can be tuned by adjusting the strain on it to obtain a precise level of tuning. In the preferred embodiment the actuator comprises a Piezoelectric actuator although any strain-producing mechanism could be used, for example a motor and lead screw.
The absolute value is preferably sensed by sensing the spacing between first and second reference points each substantially fixed relative to the fiber. As a result the absolute value is directly related to the strain on the fiber. In one preferred embodiment the first and second reference points float relative to the actuator force coupling. As a result the only error lies in the hysteresis in the fixing between the reference points and the fiber which will be negligible as the actuator force is exerted at another point. The spacing between the reference points can either be a direct linear measure of the stretching or compression of the fiber, or can be related to changes in the diameter of the fiber when it is in tension in which case the first reference point can be provided be substantially at the surface of the fiber and the second reference point is spaced from the surface of the fiber.
Preferably the sensor comprises a capacitance sensor including first and second electrodes spaced from one another and each substantially fixed relative to the fiber. Capacitance sensors are highly reliable and accurate. In one preferred embodiment the actuator exerts a force on the fiber by virtue of first and second ferrules bonded to the fiber and spaced from one another. In one preferred embodiment third and fourth ferrules are provided intermediate the actuator ferrules for limiting buckling under compression; the further ferrules are fixed relative to the fiber but float relative to the first and second ferrules and one another and carry the first and second electrodes, the electrodes preferably being provided on opposing faces of the further electrodes and a plane perpendicular to the axis of the fiber. As a result one preferred manner of obtaining an absolute measurement of the strain on the fiber is achieved, whether in compression or tension. Preferably yet further ferrules are provided intermediate the third and fourth ferrules, floating relative to the fiber carrying further electrodes on at least the end faces opposing the outer ferrules. It will be appreciated that any number of such additional ferrules can be provided, allowing an extended length of fiber to be placed under compressional tension and the electrodes on the yet further ferrules can be in electrical connection or electrically insulated.
In an alternative embodiment which is cheaper and simpler, the electrodes may be provided on the ferrules through which the actuator force is exerted, on opposing faces thereof. These ferrules may be of a bore diameter arranged to limit buckling under compression, or may be of larger bore such that only a tensioning force may be exerted, which reduces the manufacturing tolerances required. In one embodiment the first ferrule may be of a first diameter and may be received in a bore of the second ferrule of a larger diameter, the electrodes being provided on opposing axial faces. In all embodiments of the capacitance sensor arrangement the diameter of one of the electrodes may be greater than that of the other to reduce axial misalignment effects.
In an alternative preferred embodiment the actuator may comprise a variable diameter cylinder with the optical fiber wrapped around it. In that embodiment a second cylinder surrounds the first cylinder with a space in between. A first electrode is provided at the fiber surface and the second on the second cylinder inner surface. Accordingly as the cylinder diameter is varied the fiber is stretched or compressed in length, and variations in the spacing between the cylinders are a direct measure of the strain, providing yet a further accurate measurement thereof. In a preferred embodiment the first electrode is formed on the fiber itself reducing yet further any possible errors from hysteresis.
In an alternative preferred embodiment, where the variations in diameter of the fiber are measured, the first electrode is provided on the fiber surface and a second cylinder surrounds, and is spaced from the fiber surface and has a second electrode formed on its inner surface. As a result the spacing is measured providing a direct measure of the strain on the fiber. The second cylinder has first and second ends and the first electrode may comprise an electrode component provided in the region of each end, and in electrical connection, providing a useful and simple measure of the spacing across the length of the second cylinder.